1. Field of the Invention
The present invention relates to an apparatus for scanning the surface of an object with a deflected laser beam or like light beam, and more particularly to a light beam scanning apparatus wherein diffraction gratings are used for deflecting the light beam.
2. Description of the Prior Art
In recent years, apparatus for linearly scanning the surface of an object with a light beam have been placed into use for facsimile systems and laser beam printers. Various forms of scanning apparatus have been proposed for these applications. While such scanning apparatus, heretofore known, generally comprise a polygonal rotary mirror or an oscillating mirror for mechanically deflecting a light beam, apparatus have also been proposed wherein a disk type hologram scanner having a plurality of diffraction gratings are used for optically deflecting a light beam. For example, a scanning apparatus including a disk type hologram scanner is disclosed in Japanese Laid-Open Patent Application No. SHO 52-120846 based on U.S. patent application Ser. No. 673,183 filed on Apr. 2, 1976.
The disk type hologram scanner comprises a disk formed with diffraction gratings exactly identical in construction and radially arranged equidistantly along the periphery of the disk. When a laser beam is caused to strike on the diffraction gratings while the disk is in rotation, the beam is deflected by diffraction for scanning. With this apparatus the scanning locus of the light beam on the surface of the object is curved in the form of a circular arc. The apparatus of the Japanese application (No. SHO 52-12086) therefore employs a lens provided between the disk and the object for correction purposes.
Another Japanese Laid-Open Patent Application No. SHO 55-45004 proposes the use of a concave mirror having a suitable curved section, such as a circular arc or elliptical section, as a means for correcting the scanning locus into a straight line.
U.S. Pat. No. 3,922,059 and U.S. Pat. No. 4,040,737 are cited respectively to disclose a hologram for deflecting a circular scanning line and a rotary mirror.
However, such lens or concave mirrors, are difficult to make and expensive and accordingly the prior art is still seeking improvements in this field.